This grant represents a collaborative effort of investigators with expertise in marrow stem cell isolation and alveolar epithelial biology. One objective is to explore 2 novel strategies to enhance the reconstitution of the damaged alveolar gas exchange surface with intravenously administered bone marrow cells. These studies build upon our earlier work in which we demonstrated that adherent marrow cells can serve as an alternative source of alveolar epithelial progenitors during bleomycin-induced lung injury in mice. These initial observations, though promising, constitute a proof of concept, since we observed only limited replacement of recipient alveolar gas exchange surface. To apply this type of technology as a therapy for acute alveolar injury syndromes (i.e ARDS), will require, however, extensive alveolar engraftment. For one augmentation strategy, we will evaluate 4 types of purified marrow preparations that are endowed with a greater intrinsic capacity for trans-differentiation, and tissue reconstitution. The other strategy will take advantage of the central role of chemokine signaling in the homing of circulating cells to target tissues. By engaging chemokine receptors of pertinent progenitor cells as they traverse the lung, we hope to facilitate migration of cells into the alveolar space. To do this, we will first generate a profile of chemokine receptor expression in the injected cell population. Using this as a guide, we will instillate selective chemokines intra-tracheally prior to the administration of marrow cells. In the second objective of this proposal, we will extend our preliminary data showing that resident lung cells possess phenotypes similar to marow-derived stem cells. For these studies, we will use highspeed flow cytometry to identify and isolate these cells in digested lung specimens. We will employ immuno-histochemistry and in situ hybridization to localize these cells in tissue sections. Finally, we will use defined in vitro and in vivo systems to assess the capacity of these isolated lung cells to differentiate along lung or hematopoietic cell fates.